The role of vibrations on the spin decoherence of molecular qubits measured by inelastic neutron scattering

Electron spins are attractive quantum bit (qubit) candidates as they exist in a superposition of their Zeeman sublevels that can be manipulated in magnetic fields by established methods. Molecules offer highly tunable systems to study the coherence properties of such spin superpositions, with coherence times measurable up to room temperature by pulsed electron paramagnetic resonance (EPR). At low temperatures, the major source of decoherence is interactions with other spins (electronic and nuclear), and chemists have been accordingly tuning molecules to minimize spin-spin couplings. Energy dissipation by lattice vibrations dominates spin decoherence at higher temperatures, becoming especially relevant for room-temperature applications of qubits such as sensing devices. Despite their importance, measurements of the full vibrational spectrum of qubit systems remain scarce. This talk will present a series of experiments performed at the Spallation Neutron Source to probe the phonon density of states of different molecular qubits. We quantified the temperature dependence of phonons, their anharmonicity, and their dependence on structural symmetry. Neutron scattering enables us to access the low-energy acoustic modes and to study their relevance in the spin relaxation mechanisms of qubits.